Fiber combiners are critical components in the development of high power fiber amplifiers and lasers. Such fused fiber combiners are used in fiber lasers and amplifiers to combine pump and signal light into active double clad fibers. However, the coupling efficiencies of current combiners are not sufficient to permit their use in very high power amplifiers and lasers. In addition, because the signal fiber is tapered down along with the pump fibers, the resulting small core diameter of the signal fiber creates significant mismatch problems for coupling with large mode area double clad fibers. These large mode field diameter mismatches cause unacceptably high loss, particularly for very large mode area photonic crystal fiber amplifiers and lasers.
Cladding-pumped fiber amplifiers and lasers possess many desirable attributes including high efficiency, diffraction-limited beam quality ruggedness, and light weight. Their primary drawback is their susceptibility to parasitic non-linear processes, primarily stimulated Brillouin scattering (SBS), which occurs when the laser signal has a line-width narrower than a few tens of megahertz. This is due to the long interaction length of the optical signal field and the core material of the fiber. Air-clad fibers possess a higher pump-cladding numerical aperture allowing multi-mode optical pump sources to be launched into smaller claddings relative to polymer and glass-clad fibers. Smaller claddings promote increased pump overlap with the core, leading to a shorter pump absorption length allowing a shorter fiber to be used, thus reducing undesirable SBS effects. Multi-mode glass-clad pump delivery fibers have numerical apertures in the range of 0.22 to 0.28 whereas glass-clad fibers have numerical apertures in the range 0.55-0.65 permitting a pump demagnification ratio of ˜2-3×. While this may be accomplished with lenses, the traversing of multiple air-glass interfaces by the laser signal introduces optical loss and potential efficiency degradation and optical damage.
A more efficient and robust approach is to bundle a single signal delivery fiber together with multiple pump delivery fibers and then interface the bundle with a suitable double-clad active fiber wherein the signal is amplified. Efficiency requires that the cores of the double-clad and signal delivery fibers be matched while brightness conservation requires that the diameter of the pump cladding of the double-clad fiber be ˜2-3× smaller than the diameter of the bundle of pump delivery fibers. One approach has been to have the bundle of pump fibers tapered down so that the outer diameter matches that of the pump cladding of the double clad fiber. However, this results in the tapering of not only the pump fibers but also the signal delivery fiber core creating a significant mode-mismatch between the signal fiber and the core of the double clad fiber. This large mismatch results in unacceptably high loss especially for very large mode area photonic crystal fibers. If the core of the double-clad fiber is made smaller to match the tapered bundle, the non-linear threshold is decreased thus reducing the effectiveness of the device. Another approach has been to have the double-clad fiber be tapered rather than the fiber bundle. This also results in a decrease in the non-linear threshold within the tapered double-clad fiber. If the signal delivery fiber is made larger to compensate for the taper within the double-clad fiber, it likely will support multiple transverse modes hindering the effective launching of the signal.